1. Field of the Invention
The invention relates to a control device of an actuator, and particularly to a control device of an actuator that converts rotary motion of a motor into linear motion of an output shaft and outputs the linear motion.
2. Description of Related Art
A variable valve actuation mechanism that changes valve characteristics of engine valves is described in Japanese Patent Application Publication No. 2008-223705 (JP-A-2008-223705). The variable valve actuation mechanism is driven when a control shaft is displaced in a direction in which its center axis extends, so as to change the operation angle and lift amount of an engine valve according to the amount of displacement of the control shaft. More specifically, the variable valve actuation mechanism increases the operation angle and lift amount when the control shaft is displaced toward one end in the direction of extension of the center axis, and reduces the operation angle and lift amount when the control shaft is displaced toward the other end in the direction of extension of the center axis.
An actuator that converts rotary motion of a motor into linear motion of an output shaft and outputs the linear motion is connected to the control shaft of the variable valve actuation mechanism as described in JP-A-2008-223705. In the internal combustion engine including the variable valve actuation mechanism, the motor of the actuator is controlled so as to displace the control shaft of the variable valve actuation mechanism coupled to the output shaft of the actuator, in the direction of extension of the center axis, thereby to control the operation angle and lift amount of the engine valve.
A control device of the actuator in the internal combustion engine as described above is provided with a position sensor or sensors that generate pulse signals at intervals of a given rotational angle in accordance with rotation of the motor. The control device of the actuator counts the number of pulse signals generated from the position sensor(s), to provide a position count value, and grasps the position of the control shaft based on the position count value. Namely, the amount of displacement of the control shaft coupled to the output shaft of the actuator varies according to the rotational angle of the motor of the actuator; therefore, using this relationship, the control device estimates the amounts of displacement of the control shaft and the output shaft from the reference positions thereof, based on the position count value that increases and decreases in accordance to the rotational angle and rotational direction of the motor. Then, the control device detects the position of the control shaft and the position of the output shaft, based on the thus estimated amounts of displacement of the control shaft and the output shaft from the reference positions.
In the internal combustion engine as described in JP-A-2008-223705, the motor of the actuator is controlled in a feedback fashion so that the thus detected positions of the control shaft and the output shaft are made close to or equal to the positions corresponding to the target operation angle and lift amount, thereby to control the operation angle and lift amount of the engine valve.
In the meantime, when the control device of the actuator is not in operation while the engine is stopped, for example, the control device does not count the position count value. Therefore, if the control shaft of the variable valve actuation mechanism and the output shaft of the actuator are displaced in the direction of extension of the center axis for some reason while the control device of the actuator is not in operation, the displacement is not reflected by the position count value, and the actual position of the control shaft and the actual position of the output shaft do not correspond with the position count value grasped by the control device any longer.
If the actual position of the control shaft and the actual position of the output shaft fail to correspond with the position count value grasped by the control device, the position of the control shaft and the position of the output shaft are controlled based on an erroneous position count value when the internal combustion engine is operated next time, and the operation angle and lift amount cannot be appropriately controlled. As a result, in the worst case, there is a possibility of occurrence of a valve stamp, i.e., contact of the piston that has elevated to the top dead center with the engine valve that is opened.
The above-described problem that the position of the output shaft is controlled based on an erroneous position count value does not only occur to the actuator that drives the variable valve actuation mechanism as described above, but also occurs similarly in a control device of an actuator which counts a position count value based on the rotational angle of a motor, and calculates the position of an output shaft relative to a reference position thereof, based on the position count value.